The invention relates to a superconducting machine device with                a rotor supported for rotation about a rotation axis, the rotor having at least one superconducting winding with a conductor that is coupled for heat transmission to a central cylindrical cavity extending in the axial direction,        a stationary cooling unit located outside the rotor having a condenser space,and        tubular line sections extending between the center cavity of the rotor and the condenser space of the cooling unit.        
The cavity, the tubular line sections and the condenser space form a closed single-tube line system with a coolant disposed therein, wherein by way of a thermosiphon effect, coolant condensing in the condenser space reaches the central cavity through the tubular line sections and coolant evaporating in the cavity is returned to the condenser space via the line sections. A machine of this type is disclosed in DE 100 57 664 A1.
Metal oxide superconducting materials with superconducting transition temperatures Tc above 77 K are known since 1987. These materials are also referred to as High-Tc-Superconductors or HTS materials and can in principle be cooled with liquid nitrogen (LN2).
Conductors using HTS materials have been employed on an experimental basis for fabricating superconducting windings for machines. Disadvantageously, however, conventional conductors of this type have a relatively small current carrying capacity when the magnetic fields reach several Tesla. In many cases, the temperature of the conductors of such windings must be maintained at temperatures below 77 K, for example between 10 and 50 K, in spite of the high superconducting transition temperatures Tc of the employed materials, because these windings would otherwise not be able to carry significant currents in large magnetic fields. However, this temperature level is significantly higher than 4.2 K which corresponds to the boiling point of liquid helium (LHe), which is used to cool conventional metallic superconducting materials with relatively low superconducting transition temperatures Tc, so-called Low-Tc-Superconductors or LTS materials.
Preferably, cooling systems in the form of so-called cryo-coolers with a closed He compressed gas loop can be used for cooling windings with HTS conductors in the aforementioned temperature range below 77 K. In particular, such cryo-coolers operate according to the Gifford/McMahon or Sterling principle, or are formed as so-called pulsed tube coolers. Advantageously, they can immediately produce a cooling effect by pushing a button and avoid handling of cryogenic liquids. These cooling devices cool the superconducting winding only indirectly via thermal conduction to a cold head of a refrigerator (see, for example, “Proc. 16th Int. Cryog. Engng. Conf. (ICEC 16”, Kitakyushu, J P, 20-24 May 1996, Published by Elsevier Science, 1997, pages 1109 through 1129).
A similar cooling technique is also used for the rotor of an electric machine, as disclosed, for example, in the aforementioned DE 100 57 664 A1. The rotor includes a rotating winding made of HTS conductors which are located in a thermally conductive winding support. The winding support includes a cylindrical central cavity extending in the axial direction, to which tubular line sections extending from the side of the winding support are connected. These line sections are routed to a raised condenser space of a cooling unit and form in conjunction with the condenser space and the central rotor cavity a closed single-tube line system. A coolant, which circulates by way of a so-called thermosiphon effect, is disposed in the line system. Coolant condensing in the condenser space flows via the tubular line sections to the central cavity, where the condensed coolant absorbs heat due through thermal coupling to the winding support and hence also to the HTS winding. The coolant then evaporates. The evaporated coolant returns via the same line sections to the condenser space where it condenses again. The required cooling power is generated by a refrigerator engine having a cold head which is thermally coupled to the condenser space.
The return flow of the coolant is driven towards the sections of the refrigerator engine operating as the condenser by a small overpressure in the central cavity, which acts as an evaporator section. The overpressure produced by the generation of gas in the evaporator section and the condensation in the condenser space thereby causes the desired return flow of the coolant. Similar coolant flow patterns are generally known in association with so-called “heat pipes.”
The coolant is transported in the conventional machine with thermosiphon cooling from a corresponding cooling unit only by gravity, thereby obviating the need for additional pumping systems. A Machine device employed on ships or offshore installations may frequently experience a static tilt, also referred to as “trim”, of up to ±5° and/or dynamic tilt of up to ±7.5° in the longitudinal direction. Before a series of these machines can be certified for installation on a ship, a reliable cooling performance of the cooling system of such a machine device under these conditions on board of a marine vessel must be ensured. If the machine were tilted in the aforementioned manner, a region of the tubular line sections between the central rotor cavity and the cooling units may be located at a lower level than the central rotor cavity. The coolant can then no longer reach the rotor cavity and cool the rotor cavity by gravity alone, so that there would be no guarantee that the machine could be adequately cooled and would operate reliably.
Several proposals have been made to eliminate this risk:
The simplest solution is to install the machine with a tilt relative to the horizontal so that there would still be a downward slope in the thermosiphon line system in the direction towards the rotor cavity even at the largest assumed trim position or oscillation amplitude. However, such tilted arrangement is undesirable for longer machines, in particular for shipboard installation because of the increased space requirements.
Instead of a single-tube line system, where the liquids and the gaseous coolant flow through the same tube sections, dual-tube line systems can be employed when a coolant is circulated by a thermosiphon effect (see, for example, WO 00/13296 A). However, in this case, an additional tube for the gaseous coolant must be provided in the region of the hollow rotor shaft.
In principle, the coolant could also be forced-circulated by a pumping device. However, this approach requires additional equipment, in particular when the coolant must be maintained at a temperature of, for example, 25 to 30 K. Such pumping systems also experience significant thermal losses and may be unable to satisfy the service life requirements for shipboard installations with their long maintenance intervals.